In analytical chemistry, gas chromatographic techniques have become important tools in the identification of chemical sample components. The basic mechanism underlying chromatographic analysis is the separation of a sample chemical mixture into individual components by injecting the sample into a carrier fluid (preferably a carrier gas) and transporting the mixture through a specially prepared separation column.
Precise control of the carrier gas flow is essential for maintaining the stability of the analyte retention times and for the accuracy of the quantitation of the analyte. Electronic pressure control (EPC) systems are known to offer programmed control of fluid pressures in response to sense signals from one or more sensors. An example of a modern electronic pressure control system is disclosed, for example, in Klein, et al., U.S. Pat. No. 4,994,096 and U.S. Pat. No. 5,108,466, the disclosures of which are incorporated herein by reference. Klein et al. also disclose electronic pressure control of fluids in "CGC Using a Programmable Electronic Pressure Controller,"J. High Resolution Chromatography 13:361, May 1990.
A conventional gas chromatographic system may be constructed to include a particular type of injection port known as a split/splitless inlet capable of operation in either of two injection modes. FIG. 1 shows the flow paths of the carrier gas flow during a splitless injection; FIG. 2 shows the flow paths of the carrier gas flow during split injection modes. As shown in FIG. 1, carrier gas is passed through a flow controller (14) to the split/splitless injection port (12) where the injected sample mixes with the carrier gas and the mixture is directed into a separation column (18). The carrier gas flow also passes to the septum purge line (12A) while any non-analyzed mixture flow is blocked in a split line (12B) by setting a three-way valve (20) to block split flow. Alternatively, as shown in FIG. 2, the three-way valve (20) can be set to pass split vent gas flow through a split vent line (12C) to a pressure controller (24) and a split vent (25), while the septum purge flow is passed to a septum purge flow controller (21) and a septum purge vent (22).
Typically, the temperature of the column (18) is controlled according to known techniques so that the sample will separate into its components. As the carrier gas (containing the sample) exits the column (18), the presence of one or more sample constituent components is detected by a detector (not shown). The pressure of the carrier gas entering the injection port (12) is controlled by the pressure controller (24) in response to an appropriate control signal.
Certain pressure programming techniques may be used to optimize the sample separation according to the injection mode employed. In particular, the split/splitless inlet is typically operated in split or splitless injection modes with control of the column flow effected by back pressure regulation. As shown in FIG. 1, in a splitless injection mode, the inlet (12) is first configured such that only a portion of the carrier gas that enters the inlet (12) can also enter the column (18), while the remainder "sweeps" the top of the inlet (12) and exits through the septum purge line (12A). The purge control valve (20) must be set so that the carrier gas that enters the inlet liner (12L) can only exit the split/splitless inlet into the column (18). While the purge gas continues to sweep the top of the inlet (12) and passes through the septum purge line (12A), the sample may be injected into the inlet liner (12L) and vaporized. Under ideal conditions, the vaporized sample is expected to transfer to the column during an initial "hold time" (typically 30 to 90 seconds). At the end of the splitless period, the purge valve (20) is opened (as shown in FIG. 2), and any residual carrier gas/sample mixture remaining in the inlet liner (12L) is swept out through the split vent line (12B).
As shown in FIG. 2, when the inlet (12) is operated in split mode, only a portion of the carrier gas that enters the inlet will also enter the column (18). Upon injection, a portion of the sample to be analyzed is carried into the column (18) while the remainder of the sample is split and directed out through the split line (12B) to the split vent line (12C) and the vent (25). The back pressure of the carrier gas exiting the injection port (12) is controlled by the pressure controller (24) in response to an appropriate control signal.
More detailed discussion of split/splitless injection techniques can be found in the prior art, such as in M. S. Klee, GC Inlets--An Introduction, Hewlett-Packard Company, February 1990; K. Grob, Classical Split and Spritless Injection in Capillary GC, 2nd Edition, Huethig, 1988; P. L. Wylie, J. Phillips, K. J. Klein, M. Q. Thompson, and B. W. Hermann, "Improving Splitless Injection with Electronic Pressure Programming,: J High Resolution Chromatography 14:649, October 1991; S. S. Stafford, K. J. Klein, P. A. Larson, F. L. Firor, and P. L. Wylie, "Applications of Electronic Pressure Control and Pressure Programming in Capillary Gas Chromatography, "Hewlett-Packard Company, Application Note 228-141, Publication Number (43) 5091-2731E, October 1991.
In splitless injection, sample transfer from the inlet liner (12L) to the separation column (18) is not wholly effective unless the entire sample is vaporized rapidly and uniformly into a vapor cloud that is available in the inlet liner (12L) during the hold time. Splitless injection is hampered when the sample is not sufficiently vaporized, or if sample components are lost when the most volatile components of the sample vaporize rapidly, expand to fill the liner (12L), and are swept away. This phenomena (known as sample discrimination) is worsened when working quantitatively with sensitive analytes, wide boiling range mixtures, or when increasing the sample volume for greater sensitivity.
In particular, a "high-flow" splitless injection (wherein the carrier gas entering the inlet is maintained at a high flow rate, and both the septum purge line and the split vent line are active) causes the flow over the top of the inlet to attract some of the sample into the purge line, with an undesireable loss of sample from the column, and a lowered total area count. A lower total flow rate would lessen this phenomena, but such a flow rate is impractical for effecting capillary separation because the purge flow is insufficient and the operation of the pressure controller (24) is problematic.
A technique exists for reconcentrating the vapor cloud into a narrow band at the head of the column before chromatography begins, such that the vapor expansion volume and flow rate onto the column are controlled by altering the inlet pressure. The pressure program is set to effect a "pressure pulse" whereby the column head pressure is increased prior to injection, then rapidly reduced to a setpoint best suited for column flow. However, in conventional split/splitless inlets, the time required to increase the pressure during such transitions is more than desireable, and the column head pressure does not increase (i.e., "pulse") as rapidly and effectively as desired. Sample discrimination can result.
Moreover, the combination of a three-way valve (20) and its associated drive circuitry contributes to the greater parts count and the increased cost of a split/splitless inlet. Three-way valves are also not as reliable as would be desired.
Accordingly, a need exists for a chromatographic system that is operable in split/splitless injection modes, wherein the carrier fluid flow is more accurately and reliably controlled, to afford an increased total area count, with use of a simpler and less expensive arrangement of components.